1. Field of the Invention
The present invention relates to novel cosmetic compositions for topical application, for the photoprotection of the skin and/or hair against ultraviolet radiation (such compositions hereinafter simply designated "sunscreen" or "sunscreen/cosmetic" compositions), to a process for the formulation thereof and also to the use of same for the cosmetic applications indicated above.
This invention more especially relates to the aforesaid sunscreen/cosmetic compositions, having improved properties and comprising oil-in-water type emulsions (in a cosmetically acceptable vehicle or carrier) that contain, as photoprotective agents which physically block the radiation (UV reflecting and/or diffusing agents), inorganic nanopigments based on the metal oxides, especially titanium dioxide.
2. Description of the Prior Art
It is well known to this art that light radiation of wavelengths of from 280 nm to 400 nm promotes tanning of the human epidermis, and that irradiation of wavelengths of from 280 to 320 nm, i.e., UV-B, causes erythema and burning of the skin which can impair the development of a natural tan; hence, such UV-B radiation must be screened from the skin.
It is also known to this art that UV-A radiation, of wavelengths from 320 to 400 nm, which tans the skin, also adversely affects it, in particular in the event of a sensitive skin or a skin continually exposed to solar radiation.
UV-A rays cause in particular, a loss in the elasticity of the skin and the appearance of wrinkles leading to a premature aging thereof. Such irradiation promotes triggering of the erythematous reaction or enhances this reaction in certain subjects, and can even be the source of phototoxic or photoallergic reactions. Thus, it is desirable to also screen out the UV-A radiation.
Many sunscreen compositions intended for photoprotection (UV-A and/or UV-B) of the skin are known to this art, and the use of inorganic nanopigments (namely, pigments, the average size of the primary particles of which does not generally exceed 100 nm) based on the metal oxides, and especially titanium dioxide, is becoming increasingly common in light of the fact that these, when they are combined with traditional UV screening agents (principally organic compounds capable of absorbing harmful radiation), provide a very high level of protection.
For a variety of reasons, associated especially with being more pleasant to use (gentleness, emollience, and the like), the sunscreen compositions currently available most typically are oil-in-water emulsions (namely, a vehicle comprising an aqueous continuous dispersing phase and an oily discontinuous dispersed phase) into which the aforesaid nanopigments have been introduced at various concentrations, where appropriate in combination with other, traditional UV screening agents. These may be present both in the aqueous phase of the emulsion and in its oily phase (also referred to as the "fatty" phase). In these traditional emulsions, which contain, in addition, emulsifying agents (or surfactants) and optionally common cosmetic additives such as perfumes, colorants, or preservatives, the size of the globules constituting the fatty phase is generally greater than several microns.
One of the major drawbacks of the known sunscreen compositions of the above type (O/W emulsion containing nanopigments), and more especially of those containing titanium dioxide TiO.sub.2 nanopigments, is that, when applied to the skin in the form of a film, they whiten the skin which is cosmetically undesirable and generally disliked by the users. As the concentration of nanopigments in the emulsion is increased, this effect becomes more pronounced. To avoid this problem, it would naturally be possible to employ smaller amounts of the nanopigments but the resulting emulsions, which would admittedly produce films displaying acceptable transparency on the skin, would then no longer afford appropriate protection in the UV range, greatly limiting the value of same.
Moreover, another difficulty presented thereby is that the traditional sunscreen emulsions based on protective nanopigments provide, after topical application to the skin, an uneven, non-homogeneous or even crude distribution of the nanopigments on the skin, which can be detrimental to the quality of the desired global photoprotective response. This poor distribution of nanopigments on the surface of the skin is often the result of a substantial lack of homogeneity (poor dispersion of the pigment in its vehicle) in the initial emulsion itself (prior to application).
Lastly, with certain of the above-indicated sunscreen emulsions, and notwithstanding the fact that they contain emulsifying agents (or surfactants), a more or less lack of stability over time is observed, which is detrimental to their preservation once packaged (storage stability). This lack of stability manifests itself, in actual practice, in more or less marked phenomena of settling of the nanopigments within the emulsion, or even of separation between the aqueous and oily phases thereof.